The present invention relates to antennas and more particularly to a flap antenna and communications system for use on a mobile platform, such as an aerospace vehicle, terrestrial vehicle, watercraft or the like.
Commercial and military aircraft may use in-flight satellite communications to access services such as television services (DirecTV or the like, radio (XM radio or the like), high-speed Internet, telecommunications and other communications services. DirecTV is a trademark of DirecTV, Inc. in the United States, other countries or both, and XM Radio is a trademark of XM Satellite Radio, Inc. in the United States, other countries or both. A high-gain antenna mounted on the aircraft may continuously track a geo-synchronously-orbiting satellite during flight. Currently, the antenna may be either a phased-array or mechanically-scanned antenna depending on the services, features and performance requirements.
A phase-array antenna, such as an electronically scanned array (ESA) or similar antenna may scan very quickly and can be manufactured in a relatively flat and conformal package. However, the electronics for such antennas are typically expensive and the phased-array beam performance degrades rapidly with increase of scan angle. A phased-array antenna is typically only useable up to about 60 degrees scanned from the antenna's boresight. At present time, ESAs are not suitable for applications in high-frequency (Ka band or above) and wide-banded (one octave or more) communications because of technical immaturity and high cost.
A mechanically-scanned antenna may be inexpensive and provide consistent antenna beam performance independent of scan angle. However, mechanically-scanned antennas typically have relatively low scan speeds and high profiles that can result in wind loading and drag. Various types of mechanical scanning antennas in use today may utilize a Luneburg Lens Array (LLA) or a gimbaled, flat-plate antenna. The LLA is an array of four hemispherical Luneburg lens on a ground plane. The effective antenna gain is for the full height of the LLA since the antenna aperture area is doubled by use of an image created by the ground plane. The flat-plate antenna may be similar to that used for terrestrial satellite TV, but the size of the effective aperture may only need to be about half for most aircraft applications. This is because most aircraft can fly above weather, where signal degradation due to rainfall attenuation is not a factor.